1. Field of the Invention
The present invention relates to a hydraulic support device for supporting a first beam on a second beam which extends parallel thereto.
2. Description of the Related Art
There are two fundamentally different fields of use for such a hydraulic support device:
1. One of the two beams is part of a flow guide wall, for instance of a head box of a paper making machine. Here the purpose of the hydraulic support device is to maintain the flow guide wall free of flexure. The pressure of the fluid (namely, the fiber suspension) is transmitted here from the flow guide wall via the pressure cushion to the other beam. This beam is connected by flexurally elastic elements to the first-mentioned beam only at its two ends and can bend freely under the support force of the pressure cushion. Thus, no means are provided for coupling the beams at locations along the beams which are between the two ends. See for instance, Federal Republic of Germany OS 36 14 302, which is equivalent to U.S. Pat. No. 4,770,745. When the one beam, which is part of the flow guide wall, bends for any reason, the pressure of the pressure cushion is changed in such direction as to cancel the bend.
2. One of the two beams is developed as a ledge which is moveable in operation for the resilient supporting of a wire screen in the region of the double wire zone of a paper making machine; in this connection it is displaceable or swingable with regard to the other beam. In such case, the hydraulic support device acts as a resilient lift device. Any change in the pressure of the pressure cushion therefore produces a change, for instance, in the distance between the two beams and a change in the force which is applied by the ledge against the wire. Thus, no means are provided for limiting a change in the distance between the two beams while there is pressure in the pressure cushion and during pressing of the contact surfaces on the pressure cushion. See, for instance, International Application WO 89/02499.
From FIGS. 1 to 5 of the above-cited German OS 36 14 302 it is known to use a tube as pressure cushion. This tube, seen in cross section, is pressed more or less flat depending on the distance between the two beams. The distance between the two beams is generally not the same over the length of the beams, primarily because the one beam is bent relative to the other beam. This has the result that the supporting force of the pressure cushion along the length of the beams (i.e. over the width of the machine) varies relatively greatly from a desired constant value.
From FIGS. 6 to 8 of Federal Republic of Germany OS 36 14 302 it is known that the individual pressure cushion can have a bellows-like profile with side walls folded inward. In this way, it has been attempted to eliminate the aforementioned disadvantage. In other words, by this proposal it was attempted to obtain a constant value of the support force over the width of the machine despite the different distance between the two beams over the width of the machine. However, this proposal is also unsuccessful even though the width of the contact surface between the pressure cushion and the beams remains the same with different distances between the two beams. The reason for this is that the internal pressure in each inclined section of the side wall produces a component of force which represents a disturbing force acting perpendicularly on the adjacent contact surface. The amount of this disturbing force depends on the inclination of the section of the side wall and thus on the distance between the two beams. Another disadvantage of this known embodiment is that it is very difficult to produce a pressure cushion which has such inwardly folded side walls and nevertheless withstands the necessary internal pressures.